A post mixed burner is a burner in which the fuel and oxidant are injected separately from the burner. The fuel and oxidant mix and react outside the burner. Most industrial furnaces use post-mixed burners.
A number of advantages can be identified using post-mixed burners in which oxidant, comprising pure oxygen or oxygen enriched air, is supplied to the combustion zone as high velocity jets and the fuel gas is entrained into the oxidant jets. One such advantage is that the burner can be designed to be very flexible because a wide variety of flame patterns are possible. The heat transfer pattern in a furnace can be altered substantially just by changing the oxidant nozzle. Another advantage is that the circulation patterns brought about by the high velocity oxidant jets result in uniform heating of the furnace. A third advantage is that the flame can be directed so as to increase the heat transfer rate to the workload. A further advantage is that mixing of the fuel and oxidant is enhanced so as to ensure complete combustion. Yet another advantage is that the formation of nitrogen oxides is reduced with high velocity jets due to the short contact at high flame temperatures within the jet.
A recent significant advance in the field of post-mixed burners is the aspirating burner and method developed by Dr. John E. Anderson which is disclosed and claimed in U.S. Pat. No. 4,378,205 and U.S. Pat. No. 4,541,796.
A problem with post mixed burners is the attainment of good flame stability. The flame around a high velocity oxygen jet entraining gaseous fuel is easily extinguished. For conventional air burners this problem is overcome by using a burner block. Recirculation of the hot combustion products within the block serves to enhance ignition. However, if this is done with pure oxygen or oxygen-enriched air as the oxidant, the burner block would become excessively hot and melt.
In order to fully realize the advantages of using high velocity oxidant jets in a post mixed burner, it is important to stabilize the flame without destroying the integrity of the jet.
The aforesaid aspirating burner and method addresses the problem of flame stability. When operating in a hot furnace, the surrounding hot gases serve to enhance ignition. Prior to mixing and reaction with the fuel, hot furnace gases are entrained into the high velocity oxidant jets. The oxidant is heated by these hot gases to a sufficiently high temperature so that the oxidant will react upon contact with the fuel gas. When operating in a cold furnace, it is necessary to bring the temperature of the fuel up to a level at which it will react upon contact with the high velocity oxidant jet. This is accomplished by supplying about 5 to 10 percent of the total oxygen required for combustion as an annular stream around the fuel stream. The annulus oxidant and fuel react heating up the fuel stream. The high velocity oxidant stream first entrains cold furnace gas followed by the heated, partially burned fuel stream. The fuel stream is at a sufficiently high temperature so that the unburned fuel will react on contact with the high velocity oxidant.
It is desirable to have a post-mixed burner and method employing high velocity oxidant jets wherein good flame stability is attained.
Accordingly, it is an object of this invention to provide a post-mixed burner and method employing oxygen or oxygen-enriched air as the oxidant wherein the oxidant may be injected directly into the furnace zone at high velocity and wherein good flame stability is attained.